1. Field of the Invention
The present invention relates to an imaging lens system and particularly, to an imaging lens system comprising three lenses which can be preferably reduced in size and weight to be used for an image-taking device utilizing an image sensor element such as a CCD, a CMOS or the like to be mounted on a portable computer, a visual telephone, a cellular phone, a digital camera and the like.
2. Description of the Related Art
Recently, there has been a remarkable development in the multimedia industry, and there has been an increasing demand for a camera utilizing an image sensor element such as a CCD, a CMOS or the like to be mounted on a portable computer, a visual telephone, a cellular phone, a digital camera and the like. Such CCD camera needs to be mounted on a limited space. Thus, it is desirable that the camera be small in size and light in weight.
Accordingly, an imaging lens system used for such CCD camera is also necessary to be small and lightweight as well. Conventionally, the so-called one-lens system using a single lens or the two-lens system using two lenses is used as such imaging lens system.
However, although these types are extremely advantageous in terms of reducing the size and weight of the lens system, they cannot sufficiently meet the high picture quality and high resolution which are necessary for the imaging lens system in these days.
Thus, conventionally, the three-lens system using three lenses has been utilized for meeting the demands for the high picture quality and high resolution (see Japanese Unexamined Patent Publication No. 2001-75006, Japanese Unexamined Patent Publication No. 2001-83409).
Recently, especially in the field of the digital cameras and the like, there has been an increasing demand for the image-taking devices using a solid image sensor element with a higher picture quality and resolution of more than 1,000,000 pixels, which tops the conventional CIF (about 110,000 pixels) and VGA (about 300,000 pixels).
However, the conventional lens system is not yet sufficient to achieve the demands for high optical performance such as a high picture quality and high resolution through well-correcting chromatic aberration and, at the same time, to achieve further reduction of the size and weight (shortening the whole length) of the lens system itself.
In the conventional lens system, the whole length cannot be shortened so much while the back focus distance can be maintained. Thus, it is difficult to shorten the whole length of the lens system while sufficiently maintaining a high telecentricity.
The present invention has been designed to overcome the foregoing problems. An object of the present invention is to provide an imaging lens system which can achieve reduction of the size and weight while maintaining a high optical performance. Further, another object of the present invention is to provide an imaging lens system which can maintain an appropriate back focus and, at the same time, maintain the necessary telecentricity.